Uintatherium

Uintatherium Temporal range: Eocene, 56–34 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Cast of the skeleton, French National Museum of Natural History in the Paris
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	†Dinocerata
Family:	†Uintatheriidae
Subfamily:	†Uintatheriinae
Genus:	†Uintatherium Leidy, 1872
Species
†U. anceps (Marsh, 1871) †U. insperatus Tong & Wang 1981
Synonyms
Genus synonymy Uintamastix Leidy, 1872 Loxolophodon Cope, 1872 Tinoceras Marsh, 1872 Dinoceras Marsh, 1872 Ditetrodon Cope, 1885 Octotomus Cope, 1885 Elachoceras Scott, 1886 Synonyms of U. anceps Titanotherium anceps Marsh, 1871 Uintatherium robustum Leidy, 1872 Uintamastix atrox Leidy, 1872 Loxolophodon furcatus Cope, 1872 Loxolophodon pressicornis Cope, 1872 Tinoceras grande Marsh, 1872 Dinoceras mirabile Marsh, 1872 Dinoceras lacustre Marsh, 1872 Dinoceras lucare Marsh, 1873 Dinoceras laticeps Marsh, 1873 Eobasileus galeatus Cope, 1873 Dinoceras distans Marsh, 1885 Tinoceras pugnax Marsh, 1885 Uintatherium latifrons Marsh, 1885 Tinoceras vagans Marsh, 1885 Uintatherium segne Marsh, 1885 Dinoceras agreste Marsh, 1885 Dinoceras cuneum Marsh, 1885 Dinoceras reflexum Marsh, 1885 Tinoceras affine Marsh, 1885 Tinoceras crassifrons Marsh, 1885 Tinoceras hians Marsh, 1885 Tinoceras jugum Marsh, 1885 Tinoceras (Platoceras) latum Marsh, 1885 Tinoceras (Laoceras) pugnax Marsh, 1885 Elachoceras parvum Scott, 1886 Uintatherium alticeps Scott, 1886

Uintatherium ("Beast of the Uinta Mountains") is an extinct genus of herbivorous dinoceratan mammal that lived during the Eocene epoch. Two species are currently recognized: U. anceps from the United States during the Early to Middle Eocene (56–38 million years ago) and U. insperatus of Middle to Late Eocene (48–34 million years ago) China.^[1] The first fossils of Uintatherium were recovered in the Fort Bridger Basin, and were initially believed to belong to a new species of brontothere. Despite generic names being assigned, such as Edward Drinker Cope's Loxolophodon and Othniel Charles Marsh's Tinoceras, and an assortment of attempts at naming new species, Uintatherium anceps is the only valid North American member of the genus.

Taxonomy

Early history

Restoration of Edward Cope's proboscidean Loxolophodon theory from 1873

Fossils of Uintatherium were first discovered in the Bridger Basin near Fort Bridger by Lieutenant W. N. Wann in September 1870, and were later described as a new species of Titanotherium, Titanotherium anceps, by Othniel Charles Marsh in 1871.^[2] The specimen (YPM 11030) only consisted of several skull pieces, including the right parietal horn, and fragmentary postcrania.^[2] The following year, Marsh and Joseph Leidy collected in the Eocene Beds near Fort Bridger while Edward Drinker Cope, Marsh's competitor, excavated in the Washakie Basin. In August 1872, Leidy named Uintatherium robustum based on a posterior skull and partial mandibles (ANSP 12607).^[2][3] Another specimen discovered by Leidy's crews consisting of a canine was named Uintamastix atrox and was thought to have been a saber-toothed and carnivorous.^[3]

Eighteen days after the description of Uintatherium, Cope and Marsh both named new genera of Uinta dinoceratans, Cope naming Loxolophodon in his "garbled" telegram^[4] and Marsh dubbed Tinoceras.^[5] Due to Uintatherium being named first, Cope and Marsh's genera are synonymous with Uintatherium.^[2] Cope described two genera in his telegram, Loxolophodon and Eobasileus;^[4][6] the latter is currently considered separate from Uintatherium.^[2] Tinoceras was a new genus made for Titanotherium anceps by Marsh.^[5][2] Several days later, Marsh erected the genus Dinoceras.^[2] Dinoceras and Tinoceras would receive several additional species by Marsh throughout the 1870s and 1880s, many based on fragmentary material.^[5][2] Several complete skulls were found by Cope and Marsh crews, leading to theories like Cope's proboscidean assessment.^[6][7] Because of Cope and Marsh's rivalry, the two would often publish scathing criticisms of each other's work, stating their respective genera were valid.^[2] The trio would name 25 species now considered synonymous with Marsh's original species, Titanotherium anceps, which was placed in Leidy's genus, Uintatherium.^[2]

Holotype skull (IVPP V6379) of U. insperatus, Paleozoological Museum of China

Many additional discoveries of Uintatherium have since occurred, making it one of the best-known and popular American fossil mammals.^[8][2] Princeton University launched expeditions to the Eocene beds of Wyoming in the 1870s and 1880s, discovering several partial since skulls and naming several species of uintatheres that are now considered synonyms of U. anceps.^[9][2] Major reassessment came in the 1960s by Walter Wheeler, who synonymized and redescribed many of the Uintatherium fossils discovered during the 19th century^[2] A cast of a Uintatherium skeleton is on display at the Utah Field House of Natural History State Park. A skeleton of Uintatherium is also on display at the Smithsonian National Museum of Natural History in Washington, DC.^[10] A new species was named based on almost intact skull, U. insperatus, found in the lower part of the Lushi Formation of the Lushi Basin in Henan Province, China.^[1]

Classification

Though initially regarded by Marsh as a brontothere,^[2] Uintatherium was recognised by Henry Fairfield Osborn in 1881 as part of the order Dinocerata. At the time, dinocerates were believed to be part of Amblypoda, a group uniting an assortment of basal ungulates from the Palaeogene,^[11][12] and were sometimes referred to simply as "dinoceratous amblypods".^[13] The group Amblypoda has since fallen out of use, and is generally regarded as polyphyletic, meaning that it was an unnatural group consisting of an assortment of distantly related clades.^[14] Dinocerata, however, has persisted, though the precise relationships of the order have been the subject of debate. Relationships with South American native ungulates (SANUs), specifically xenungulates, have been suggested,^[15][16][17] with Spencer G. Lucas and Robert M. Schoch in 1998 supporting the complete removal of both clades from Ungulata.^[18] If dinoceratans and xenungulates are indeed related, they may constitute the mirorder Uintatheriamorpha.^[15][18] However, it has been stated that no strong evidence for this relationship exists, and that similarities may simply be the result of convergent evolution.^[19][20]

Dinocerata has historically been divided into two families: Prodinoceratidae, and Uintatheriidae.^[15][16] The latter family consists of the majority of dinocerate genera,^[15] and has itself been divided into Gobiatheriinae and Uintatheriinae;^[21] occasionally, the latter has been divided even further, down to tribe level (Bathyopsini and Uintatheriini)^[15] Walter H. Wheeler suggested in 1961 that the taxa now classed as uintatheriines formed a primarily anagenetic lineage, and that Uintatherium was one of few diverging genera, possibly evolving from Bathyopsis middleswarti (which he believed to be ancestral to both Uintatherium and later dinocerates).^[2] Robert M. Shoch and Spencer G. Lucas, in 1985, performed a phylogenetic analysis of Dinocerata, and recovered Uintatherium as the sister taxon to a clade consisting of Eobasileus and Tetheopsis, slightly more derived than Bathyopsis.^[15]

Description

Restoration

Uintatherium was a large browsing animal, with U. anceps standing 1.5 m (4 ft 11 in) at the shoulder^[22] and having a body mass of 3,000–4,500 kg (6,600–9,900 lb);^[18] the size of U. insperatus is not certain, though it is believed to have been smaller.^[1] Despite its size, U. anceps was exceeded in sized by related taxa such as Eobasileus. Uintatherium as a whole appears to have exhibited strong sexual dimorphism: males had larger canines, larger flanges on the lower jaws, larger sagittal crests, larger horns, and an overall larger body size.^[15]

Skull

Cast of U. anceps skull, French National Museum of Natural History, Paris

The skull of Uintatherium was roughly three times longer than it was wide.^[22] Most skulls range from 69–85 cm (27–33 in) in length.^[18] Some specimens had skulls which, when measured at the zygomatic arches, were roughly 32 cm (13 in) wide, suggesting a very large overall skull size.^[1] Furthermore, some specimens initially referred to Loxolophodon have skull lengths of up to 91 cm (36 in), nearly a third larger than most others.^[11] Uintatherium skulls can be distinguished from those of other uintatheriins (if that clade exists) by the broadness of their skulls. Eobasileus and Tetheopsis have skulls which are relatively longer and slenderer.^[15]

The nasal bones of Uintatherium were very long, comprising roughly half of the total length of the skull. They project distinctly, such that they completely overhang the external nares. At the front of each nasal was a small bony protrusion, which differed in form between specimens: in some, they were small and deflected upward and outward, while in others, they were larger and more horizontal.^[23] Uintatherium had large zygomatic arches, of which the maxilla comprised the anterior portion, similar to proboscideans. Like other dinoceratans, the skull of Uintatherium lacked a postorbital process.^[11] At the back of Uintatherium's skull was a very large occipital crest, extending posteriorly (rearward) further than the occipital condyles.^[23] To either side of the occipital crest sat a pair of very large parasagittal crests.^[18] In some specimens, the lacrimal, the above the orbits (eye sockets), was distally (outwardly) expanded, overhanging the zygomatic arches; in others, those formerly referred to Loxolophodon, the zygomatic arches projected beyond them.^[11] Much like other dinoceratans, Uintatherium's skull was adorned with a series of well-developed cranial outgrowths,^[24], sometimes called horns,^[15] three pairs in total. Those above the maxilla sat directly above the diastema (gap) separating the canines and premolars, and the so-called parietal horns sat far anterior to (in front of) the occipital bone,^[15] on the parasagittal crests.^[18] This differs from the related Eobasileus and Tetheopsis, in which the parietal horns were closer to the occipital. Furthermore, in those two genera, the premaxillary horn sat above the premolars, meaning the portion of the snout anterior to the maxillary horns was far longer; in Uintatherium, the snout anterior to the maxillary horns was fairly short.^[15] Contrary to their occasional description as horns, it is unlikely that any of these outgrowths were cornified (reinforced by keratin), and they were likely covered only by skin. Uintatherium's skull was lightened by well-developed sinuses, similar to other animals with extensive cranial ornamentation, though not to the same extent.^[24]

Uintatherium's brain appears to have been among the smallest, proportionally of any mammal, such that Othniel Charles Marsh noted that "it could apparently have been drawn through the neural canal of all the pre-sacral vertebrae".^[23] While an elephant-like trunk or proboscis was suggested early on, based on alleged affinities to proboscideans,^[7] the structure of the ethmoturbinal bones of the nasal passage and the structure of the olfactory nerves suggest that no such structure existed.^[23] In its place, there may have been a flexible upper lip, analogous to that of modern rhinocerotids.^[18]

Mandible of Uintatherium, minus the lower incisors

Projecting from the anteroventral (towards the front and at the bottom) portion of Uintatherium's mandibles (lower jaws) were a pair of large flanges. In most specimens, these would have provided support to the large upper canines,^[15] though specimens formerly referred to Loxolophodon had smaller flanges which did not extend as far.^[11] It has been suggested that the observed difference in flange size is the result of sexual dimorphism, with larger-flanged jaws belonging to males.^[23] Similar structures are observed in the related Bathyopsis.^[12] Flanges aside, the lower jaw of Uintatherium was fairly slender. Unlike most other ungulates, the condyles were deflected posteriorly, likely to accommodate the large upper tusks: without such a modification, the jaws would be unable to fully open. This condition is otherwise only seen in some marsupials and members of the former order Insectivora. The mandible's coronoid process is large, curves posteriorly, and is pointed dorsally (at the top).^[23]

Dentition

Upper and lower cheek teeth of Uintatherium

Uintatherium had a dental formula of 0.1.3.33.1.3.3,^[11][24][a] though one early record provided a dental formula of 0.1.3.33.1.4.3.^[23] Uintatheriids in general lacked upper incisors, and Uintatherium was no exception.^[11][15] The loss of the upper incisors likely indicates the presence of a firm elastic pad on the ventral portion of the premaxilla, similar to that of ruminants. The lower incisors were bilobate, bearing crowns which were split into two distinctive cusps.^[24] The lower canines were somewhat incisiform, meaning that they resembled conventional incisors,^[15][23] while the upper canines were large and have been compared to sabres. Eobasileus and Tetheopsis have similar canines. In life, these large canines would have been projected by the large flange projecting from the mandible.^[15] Between the canines and cheek teeth, there was a large gap, the diastema.^[15][23] Behind the diastema were three upper premolars and three upper molars, all of which were fairly small.^[11][23] All of Uintatherium's cheek teeth were brachyodont, meaning they had short crowns and well-developed roots.^[23] The first upper premolar appears to have completely disappeared, with only the ovvasional preservation of the alveolus (tooth socket);^[23] reduced first premolars, on both upper and lower jaws, are a diagnostic trait of dinoceratans.^[18] Whether or not the first lower premolar was retained is uncertain, as some sources report it as present,^[11] while others report it as absent.^[23] The third lower molar was very short, with reduced ectoconid and hypoconulid crests. The paraconids and paracristids of all teeth from the third upper premolar to the second upper molar were greatly reduced. As a whole, it has been noted that Uintatherium's dentition is intermediate between that of Bathyopsis and Eobasileus: the former taxon has smaller upper canines, less incisiform lower canines, and less bilophodont cheek teeth than Uintatherium, while the latter has more extreme developments of those traits. This is part of the reason why an evolutionary sequence between the three genera has been proposed.^[18]

Paleoecology

Contemporary fauna

Uintatherium anceps

A map of North America during the Eocene, with modern borders shown

U. anceps is known from various strata from the Bridgerian and Uintan North American land mammal ages. This corresponds to the interval between 50.5 and 39.7 million years ago—a span of just over 10 million years within the Eocene. The oldest remains confidently assigned to this species are from the faunal zone "BR3" of the Bridger Formation, which is at the end of the Bridgerian land mammal age.^[25]

In the Bridger Formation, U. anceps coexisted with a variety of primitive ungulates including helohyids, homacodontids, brontotheriids, amynodontids, and hyopsodontids. The environment was also host to some of the ancestors of modern perissodactyl groups including Hyrachyus (a primitive relative of rhinos), Helaletes (an early relative of tapirs), and several species of Orohippus (a primitive horse). North America at the time also had a diverse assemblage of early primates including Microsyops, Notharctus, Smilodectes, and the members of Omomyidae (relatives of modern tarsiers). Mammalian predators of the region included mesonychids like Mesonyx and Harpagolestes, hyaenodontids like Limnocyon and Sinopa, oxyaenids like Patriofelis and Machaeroides, and early carnivoran-relatives like Miacis and Vulpavus. A variety of more enigmatic mammal forms were also present including members of Tillodontia, Stylinodontidae, and Pantolestidae and the small insectivorous Apatemys and Metacheiromys. Primitive sciuromorph rodents, leptictids, and eulypotyphlans coexisted with the metatherians Herpetotherium and Peradectes.^[26][25]

Reptiles were also abundant in this environment. Fossils from turtles including softshelled turtles, tortoises, terrapins, and baenids lived alongside anguids, varanids, teiids, and boids as well as crocodilians like Boverisuchus and Borealosuchus. Remains of primitive owls and cranes have also been found.^[26][27]

A pair of Uintatherium depicted with the contemporary equid Orohippus

In the transition from the Bridgerian to the Uintan, several of these animals became extinct and new forms emerged. The oxyaenids and phenacodontids disappeared during this transition and new groups like the oromerycids and the earliest chalicotheres (the eomoropids). This transition is followed by the appearance of several medium and large ungulate genera including Protylopus, Amynodon, and Eobasileus. This faunal subinterval is represented by the Devil's Graveyard Formation and has been argued to be a distinct land mammal sub-age (the "Shoshonian" or "UI1b biochronological zone"), although this is not universally accepted. This transition also saw a marked decline in primate diversity in North America, which would continue throughout the Eocene until primates eventually became extinct in North America.^[25][28]

The middle-Uintan land mammal age (sometimes called "UI2" biochronological zone) is the most recent interval from which fossils of U. anceps are known. This corresponds to the eponymous Uinta Formation. This interval saw the diversification of brontotheres, helohyids, and rhinocerotoids as well as the emergence of the first protoceratids, agriochoerids, and camelids. It also saw the extinction of North American cimolestans and leptictids as well as most of the remaining North American primates, with only leporids and omomyids remaining extant. Primitive carnivoramorphs like Miocyon also emerged. The end of this interval saw the final extinction of Uintatherium in North America alongside other long-lived genera such as Mesonyx and Hyrachyus.^[25][29]

Uintatherium inseperatus

A map of the world during the Middle Eocene, with modern borders shown

The second species of Uintatherium, U. inseperatus, lived in the Lushi Formation what is now Henan, China during the Middle Eocene.^[30] The precise age of the fossils assigned to this species are uncertain, but they have been estimated to be between 48 and 37 million years ago, which is roughly contemporaneous with the existence of U. anceps in North America.^[31] This corresponds to the Sharamurunian Asian land mammal age, which lasted for about the same length of time.^[32] Remains assigned to U. inseperatus have also been found in the similarly-aged Uqbulak Formation in the Junggar Basin.^[33]

The composition of Asian land mammal assemblages was similar in several ways to the contemporary assemblages in North America, although the precise timing of faunal turnover is not as well studied with respect to Eocene ecosystems in Asia. The carnivorous mammals of the continent were generally similar, with mesonychids, haplodectids, hyaenodontids, and the carnivoramorphan Miacis being the most abundant predators. However, several endemic carnivores coexisted with these including Eusmilus (an early nimravid), Cynodictis (a primitive amphicyonid), and the controversial carnivorous ungulate Andrewsarchus. Prey for these animals included a diverse array of terrestrial ungulates including late surviving members of Paleocene lineages such as the coryphodont Eudinoceras, dichobunids, tillodontians, and taeniodontans. Ungulate groups common in North America were also represented, including Hyrachyus as well as the helohyids, brontotheriids, helaletiids, and amynodontids. They were accompanied by a diverse array of perissodactyls, which underwent a radiation in Asia during the Middle Eocene. These new groups included the paraceratheriids, hyracodontids, chalicotheriids, and deperetellids. The artiodactyl anthracotheres also first evolved in Asia during this period.^[34]

Notes

1. No incisors, one canine, three premolars and three molars in each half of the upper jaw, and three incisors, one canine, three premolars and three molars in each half of the lower jaw, resulting in 34 teeth in total

References

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15. Shoch, Robert M.; Lucas, Spencer G. (1985). "The phylogeny and classification of the Dinocerata (Mammalia, Eutheria)" (PDF). Bulletin of the Geological Institutions of the University of Uppsala, N.S. 11: 31–58.
16. Spencer G. Lucas, Robert M. Schoch 19. "Dinocerata" in: Evolution of Tertiary Mammals of North America: Volume 1, Terrestrial Carnivores, Ungulates, and Ungulate like Mammals (1998)
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Further reading

• Paleontology portal

• Academy of Natural Sciences
• National Park Service
• Wood, Horace Elmer 1923, The problem of the Uintatherium molars, Bulletin of the American Museum of Natural History; v. 48, article 18
• Fossil Evidence – Eocene: Smithsonian National Museum of Natural History